1. Field of the Invention
The present invention relates to a surface acoustic wave filter in which first and second longitudinally coupled resonator surface acoustic wave filter units are connected in a cascade arrangement, and more particularly, to a balanced-type surface acoustic wave filter having a balanced-to-unbalanced conversion function.
2. Description of the Related Art
With the miniaturization of mobile telephones, RF stages including band-pass filters are required to be reduced in size. Recently, balanced-type surface acoustic wave filters covering high frequencies and having a balanced-to-unbalanced conversion function have been used as such band-pass filters.
An output terminal of a balanced-type surface acoustic wave filter having the balanced-to-unbalanced conversion function is connected to a mixer IC with balanced input and output or differential input and output, that is, to a balanced-type mixer IC. The balanced-type mixer IC reduces the effect of noise and stabilizes the output. Accordingly, such balanced-type mixer ICs are currently in widespread use so as to improve characteristics of mobile telephones.
The impedance of a surface acoustic wave filter used for an RF stage is typically about 50Ω. On the other hand, the input impedance of a known balanced-type mixer IC is typically in the range of about 150Ω to about 200Ω. Accordingly, in a surface acoustic wave filter having the balanced-to-unbalanced conversion function, the ratio of the impedance of an unbalanced signal terminal to the impedance of a balanced signal terminal is often set so that it falls within a range of about 1:3 to about 1:4.
An example of a surface acoustic wave filter having the balanced-to-unbalanced conversion function is described in Japanese Unexamined Patent Application Publication No. 11-97966 (Patent Document 1). FIG. 15 is a schematic plan view showing an electrode configuration of a balanced-type surface acoustic wave filter described in Patent Document 1.
In a surface acoustic wave filter 501, a first longitudinally coupled resonator surface acoustic wave filter unit 511 is connected to an unbalanced signal terminal 502. The first longitudinally coupled resonator surface acoustic wave filter unit 511 includes IDTs 512, 513, and 514 disposed along a propagation direction of a surface acoustic wave. The center IDT 513 is connected to the unbalanced signal terminal 502. Reflectors 515 and 516 are disposed on either side of a region including the IDTs 512 to 514 in the propagation direction of the surface acoustic wave.
On the other hand, a second longitudinally coupled resonator surface acoustic wave filter unit 521 is connected to the first longitudinally coupled resonator surface acoustic wave filter unit 511. The second longitudinally coupled resonator surface acoustic wave filter unit 521 includes IDTs 522, 523, and 524 which are disposed in the propagation direction of a surface acoustic wave, and reflectors 525 and 526. The IDTs 522 and 524 are electrically connected to the IDTs 512 and 514 via signal lines 505 and 506, respectively. The center IDT 523 is divided in the propagation direction of the surface acoustic wave, whereby a first IDT portion 523a and a second IDT portion 523b are provided. The IDT portions 523a and 523b are electrically connected to a first balanced signal terminal 503 and a second balanced signal terminal 504, respectively.
Thus, in the surface acoustic wave filter 501, the first longitudinally coupled resonator surface acoustic wave filter unit 511 and the second longitudinally coupled resonator surface acoustic wave filter unit 521 are connected in a cascade arrangement. Patent Document 1 describes that the impedance of the balanced signal terminals 503 and 504 is about 200Ω. That is, the balanced-type surface acoustic wave filter 501 having the above-described impedance ratio of about 1:4 is disclosed.
On the other hand, Japanese Unexamined Patent Application Publication No. 9-321574 (Patent Document 2) discloses a surface acoustic wave filter that does not have the balanced-to-unbalanced conversion function. As shown in FIG. 16, in a surface acoustic wave filter 601 described in Patent Document 2, a first longitudinally coupled resonator surface acoustic wave filter unit 602 and a second longitudinally coupled resonator surface acoustic wave filter unit 603 are connected in a cascade arrangement. The first longitudinally coupled resonator surface acoustic wave filter unit 602 is connected to an input terminal 604. The second longitudinally coupled resonator surface acoustic wave filter unit 603 is connected to an output terminal 605.
In Patent Document 2, different electrode finger cross widths are set to the first longitudinally coupled resonator surface acoustic wave filter unit 602 and the second longitudinally coupled resonator surface acoustic wave filter unit 603, whereby an input impedance of about 50Ω and an output impedance of about 150Ω are set. That is, a surface acoustic wave filter having the impedance ratio of about 1:3 is provided in which two elements are connected in a cascade arrangement.
Currently, a balanced-type mixer IC having an input impedance of about 100Ω is increasingly used as a balanced-type mixer IC connected to a stage subsequent to the above-described surface acoustic wave filter. Accordingly, a surface acoustic wave filter used in an RF stage is required to have a low output impedance. That is, a surface acoustic wave filter having a ratio of the impedance of an input terminal to the impedance of an output terminal of approximately 1:2 and having the balanced-to-unbalanced conversion function is required.
In the surface acoustic wave filter 601 described in Patent Document 2 which does not have the balanced-to-unbalanced conversion function and in which two elements are connected in a cascade arrangement, the ratio of an input impedance to an output impedance of about 1:3 is set by increasing the electrode finger cross width of the first longitudinally coupled resonator surface acoustic wave filter unit 602 and decreasing the electrode finger cross width of the second longitudinally coupled resonator surface acoustic wave filter unit 603. However, in such a configuration, an impedance mismatch is likely to occur at a cascade connection point. This causes an increased passband insertion loss.
In the balanced-type surface acoustic wave filter 501 described in Patent Document 1, the fifth IDT 523, which is a center IDT in the second longitudinally coupled resonator surface acoustic wave filter unit 521, is divided into two portions, the IDT portions 523a and 523b. The IDT portions 523a and 523b are connected in series. Inevitably, the impedance of the balanced signal terminals 503 and 504 increases. Accordingly, the impedance ratio of about 1:4 is set as described above. In such a configuration, the impedance ratio may be controlled by changing electrode finger cross widths as described in Patent Document 2. However, in order to achieve the ratio of an input impedance to an output impedance of about 1:2 by using the method of changing electrode finger cross widths, the electrode finger cross width of the IDT 523 connected to the balanced signal terminals 503 and 504 is required to be about twice the electrode finger cross width of the IDT 513 connected to the unbalanced signal terminal 502. This not only increases the size of the electrodes but also increase the size of the entire surface acoustic wave filter 501.